Use %s and %b in GitCommitMessageParserWorker, not %B

Summary:
See T129, some older git doesn't have %B and we can reasonably fake it with %s
and %b.

Test Plan:
Reparsed all of the Phabricator repository with this worker, commit messages
look fine.

Reviewed By: rm
Reviewers: rm, aran, jungejason, tuomaspelkonen
CC: aran, rm
Differential Revision: 209